Human diploid cells to young (IMR 91 or WI-38 at low passage number) and old (GM 731 or WI-38 at high passage number) age grown to confluency in 16 oz. prescription bottles will be labelled with 32P. Transfer RNA will be isolated and separated by two dimensional gel electrophoresis. Phenylalanine tRNA (or other tRNAs) isoacceptors will be identified by aminoacylation of the separated tRNA species with 3H-phenylalanine and quantitatively measured by their 32P or 3H counts. Base composition and location of changes in these tRNA species will be analyzed. Data obtained from these studies with young and old cell will be compared.